Exchange 2010 - Room mailbox working hours

To define the working hour of a room mailbox, you can use powershell with the command Get-MailboxCalendarConfiguration and Set-MailboxCalendarConfigurationbut I found out in a comment on another blog I found that you can do it using the OWA/ECP with an account with enough rights.

Open exchange ECP at mail.yourocmpany.com/ECP, log in as high privileges users (I used my domain admin account)

Click on top on "manage my organization" and choose "Another users"

In the pop-up box search for the room mailbox you want to manage and then go to settings, Calendar, there you can define the working hours and then click save in the bottom right and you're done.